Insulin promotes human muscle protein anabolism by inhibiting proteolysis. It has been proposed that elevating the plasma amino acid concentrations would allow expression of an effect of insulin to stimulate protein synthesis. To examine the effects of elevating plasma amino acids with or without supplemental insulin on human muscle protein turnover, we will measure phenylalanine (PHE) balance (BAL) and H-PHE kinetics in both forearms of postabsorptive, healthy adults during a continuous H-PHE infusion.